The present invention relates to a mold for plastic molding having improved resistance to corrosion by halogen gases, and a steel for the preparation thereof.
It is desirable to control or reduce the combustibility of plastic products. A test method for determining combustibility is prescribed in Underwriters' Laboratories Standard (UL-94V).
Efforts have been made to impart flame-retardability or self-extinguishability to thermoplastic resins of low softening point and high fluidity by incorporating therein a halogenated (chlorinated or brominated) aliphatic or aromatic hydrocarbon together with a phosphoric acid ester or an antimony compound. In the thermoplastic resin containing flame-retarding agent, a part of the flame-retarding agent is decomposed during injection molding to evolve a halogengas or gaseous halogen compound which causes pitting on the cavity walls of the mold. Tough steels such as SAE 1055 and SAE 4135 have been employed as steels for molds used in plastic molding. However, these steels are not resistant to pitting. Although chromium plating is effected to prevent pitting, this technique is not so effective, since the corrosive gas permeates into microvoids of the coating layer to corrode the base steel. Employment of a stainless steel such as SAE 30304 has also been attempted, but stainless steel is unsatisfactory with respect to strength and workability. Under the above described circumstances, a strong need has been felt for a mold for plastic molding having an excellent resistance to corrosion by halogen gases or halogen compound and for a steel for the preparation thereof having a high toughness and workability.